1. Field of the Invention
The present invention relates to a control circuit and in particular to a control circuit for speed and rotational direction of a fan, which produces the effect of cost reduction and an increase in the layout space.
2. Description of Prior Art
Motor technology plays a critical role in the development of a modern automated technology. Specially applied to the fan, the motor drives the fan to rotate and stimulate airflow, achieving the effect of heat dissipation. Therefore, it is quite suitable for heat dissipation of electronic products in electronic industry.
Please refer to FIG. 1, which is a control circuit of the prior art for a three-phase DC brushless motor. The control circuit includes a signal conversion unit 10, a direction determining unit 11, a microprocessor 12, and a drive unit 14. The signal conversion unit 10 receives a PWM (Pulse Width Modulate) signal at its input end and converts the PWM signal into a DC level signal. The DC level signal is then transmitted to the rotational speed pin 121 connected to the microprocessor 12 through the output end of the signal conversion unit 10 such that the microprocessor 12 generates a control signal based on the received DC level signal and the drive unit 14 drives and controls the rotational speed of the fan 16 based on the received control signal.
The above-mentioned direction determining unit 11 receives an input signal at its one end and the other end thereof is connected to the rotational direction pin 122 of the microprocessor 12 such that the direction determining unit 11 performs the determining process based on the received input signal. If the input signal is determined to be clockwise, a clockwise signal is generated and transmitted to the microprocessor 12 such that the microprocessor 12 generates another control signal to the drive unit 14 based on the received clockwise signal to make the drive unit 14 drive and control the clockwise rotation of the fan 16 based on the received control signal. If the input signal is determined to be counter-clockwise, a counter-clockwise signal is generated and transmitted to the microprocessor 12 such that the microprocessor 12 generates another control signal to the drive unit 14 based on the received counter-clockwise signal to make the drive unit 14 drive and control the counter-clockwise rotations of the fan 16 based on the another received control signal.
Though the control circuit of the prior art for the three-phase DC brushless motor can achieve the effect of controlling the speed and rotational direction of the fan 16, another problem takes place. This problem is that when we control the speed and rotational direction of the fan 16, two independent circuits (i.e., the signal conversion unit 10 and the direction determining unit 11) are required, each providing a set of signals (i.e., two sets of signals totally) to the microprocessor 12 such that the microprocessor 12 is required to provide a pin (i.e. the above-mentioned rotational speed pin 121) to specifically receive the signal for rotational speed and to provide another pin (i.e., the above-mentioned rotational direction pin 122) to specifically receive the signal for rotational direction. As a result, the layout space on the PCB is reduced and the cost is then increased.
From the above description, the prior art has the follow disadvantages:                1. The cost is increased.        2. The layout space on the PCB is reduced.        3. Two pins of the microprocessor 12 are required for two independent circuits and thus one pin of the microprocessor 12 is not available for another circuit.        
Therefore, how to overcome the above problems and disadvantages is the focus which the inventor and the related manufacturers in this industry have been devoting themselves to.